Heat Hardening of a Larval Amphibian is Dependent on Acclimation Period and Temperature

Abstract

The thermal tolerance-plasticity trade-off hypothesis states that acclimation to warmer environments increases basal thermal tolerance in ectotherms but reduces plasticity in coping with acute thermal stress characterized as heat hardening. We examined the potential trade-off between basal heat tolerance and hardening plasticity, measured as critical thermal maximum (CTmax) of a larval amphibian, Lithobates sylvaticus, in response to differing acclimation temperatures (15{degrees} and 25{degrees}C) and periods (3 or 7 days). A hardening treatment applied 2 hours before CTmax assays induced pronounced plastic hardening responses in the cool, 15{degrees}C treatment after 7 days of acclimation, compared to controls. Warm acclimated larvae at 25{degrees}C, by contrast, exhibited minor hardening responses, but significantly increased basal thermal tolerance. These results support the trade-off hypothesis and fill a knowledge gap in larval amphibian thermal plasticity. Elevated environmental temperatures induce acclimation in heat tolerance yet constrains ectotherm capacity to cope with further acute thermal stress. Summary StatementA larval amphibian follows the trade-off hypothesis such that the group with the highest basal heat tolerance displays the lowest hardening response and vice-versa.